A late life opportunity: using a multidisciplinary approach to unlock reserves in the Rannoch Formation, Ninian Field

Abstract

Over 10 9 BBL oil have been produced from the Ninian Field. Although the Rannoch Formation is known to contain 10% of the field STOIIP, yet less than 5 × 10 6 BBL of production can be allocated to this formation. The low permeability Rannoch Formation produced low watercut oil at low oil production rates. Other, higher permeability, Brent Group zones produce at higher rates and have been preferentially perforated. Unable to contribute to production when higher permeability reservoir zones were co-produced, the Rannoch Formation became enclosed by highly swept reservoir zones. This lack of production contribution indicated a requirement for dedicated production from the Rannoch Formation. However, historical production rates were uneconomic. Any strategy for economic exploitation of the Rannoch Formation was partly hindered because the distribution of productive sands in the Rannoch Formation was not previously understood, rendering the production performance unpredictable. In addition, this formation is seismically unresolvable. A sedimento- logical reappraisal of the Brent Group resulted in a depositional facies scheme. This provided a correlatable framework within which the depositional context and heterogeneity of the Rannoch Formation could be understood. This formation predominantly comprises middle shoreface sandstones with subordinate higher permeability upper shoreface facies. Cyclic stacking of these deposits defines parasequences with a clinoform geometry. A scoping sector simulation model was built to assess the Rannoch Formation production capability. This concluded that a horizontal well, combined with artificial lift, would deliver maximum, economic, production. The well was to be maintained within productive upper shoreface sands, c. 10′ thick and within the Rannoch Formation interval, c. 40′ in thickness, and drilled utilizing a joint geosteering and biosteering approach. The well penetrated 490′ of productive upper shoreface sand within a 1700′ horizontal section, flowed at economic rates and is predicted to produce 10% more oil than the basis on which the well was justified.